The present invention relates to a reactor core, and more particularly, to a reactor core suitable for the use in a Boiling Water Reactor having an apparatus for adjusting cooling water flow rate.
A fuel assembly used for a Boiling Water Reactor (BWR) includes a plurality of fuel rods and a channel box. Each of the fuel rods is filled with a plurality of fuel pellets including fissile material in the cladding tube, are bundled in a square lattice form. The square channel box encloses the bundled fuel rods. The square channel box has an outer width of about 14 cm and a square cross-section. A core, which is disposed in the reactor pressure vessel of a BWR, is loaded with a plurality of fuel assemblies internally. For nuclear fuel material that is used to a fuel pellet, enriched uranium or plutonium-enriched uranium is used in a chemical form of oxide. Because fuel rods are heated by the heat generated by the nuclear fission of the nuclear fuel material, they are cooled by cooling water (coolant) which is light water supplied to the core. The cooling water is circulated by pump.
Thermal margin is expressed by a minimum critical power ratio (MCPR) and is defined by a value obtained by dividing fuel rod power at which cooling water transits to a film boiling state on the surface of the cladding tube of the fuel rod and heat removal efficiency starts to greatly decrease, by actual reactor power. The fuel rod power is critical power. It is necessary to maintain the thermal margin so that it is equal to or more than a designated value according to design criteria while the reactor is in operation and in a transient state. The thermal margin decreases as core power density increases.
When a reactor is in rated operation, thermal power of fuel assemblies is the lowest in the outermost layer of the core where a large quantity of neutrons leaks. For this reason, in conventional technology, pore diameter of an orifice of the cooling water inlet disposed in the outermost layer region of the core is designed such that the pore diameter is smaller in the inner region than that in the outermost layer region, thereby increasing inflow resistance of the cooling water. In the region of the core other than the outermost layer region, that is, in the inner region where power of fuel assemblies relatively becomes high, the flow rate of the cooling water increases and thermal margin of the core during rated operation can be ensured.
Japanese Patent Laid-open No. Hei 7(1995)-181280 describes the adjustment of the pore diameter of the orifice in the cooling water inlet. In the conventional technology, the core is divided into two regions, inner and outer regions, excluding the outermost layer region, and the pore diameter of the orifice in the coolant inlet located in the region on the outer side of 70% of the core radius in the radial direction is made smaller than the pore diameter of the orifice located in the region (central region) on the inner side of the outer region. To do so, flow rate of cooling water in the region on the inner side of 70% of the core radius where power is relatively high increases, thereby increasing thermal margin of the core. In an embodiment described in Japanese Patent Laid-open No. Hei 7(1995)-181280, fuel assemblies loaded in the core are divided into two groups according to in-core fuel dwelling time, and two types of orifices for the coolant inlets are used to efficiently improve thermal margin. However, in the conventional technology, from a perspective of maximization of thermal margin during rated operation, nothing is considered about the increase in power of the fuel assembly associated with the increase in the flow rate of the cooling water, which will be described later in this document. Further, the division of the core region and the setting of the pore diameter of the orifice in the cooling water inlet are not completely optimized.